Showing papers in "Philosophical Magazine Part B in 1981"

An asymmetric double-well potential model for structural relaxation processes in amorphous materials.

Abstract: Peaks in the ultrasonic attenuation observed in amorphous materials are almost always explained in terms of a classical, thermally activated, structural relaxation process represented by a symmetric double-well potential with a broad distribution of barrier heights This model suffers from two drawbacks: a low barrier cut-off is required to produce a peak in the attenuation and the magnitude of the attenuation does not scale with the frequency of the acoustic waves, contrary to experimental evidence This paper presents a model based on an asymmetric double-well potential having distributions of both the barrier height g(V) and the asymmetry f(Δ) Such a model is justified by the random local environments in an amorphous material, and has the advantage that no low barrier cut-off is required, in contrast with the symmetric double-well model To illustrate the predictions of the model specific forms are assumed for the distribution functions: f(Δ) is taken as constant (valid for temperatures well

Photoluminescence and optical properties of plasma-deposited amorphous Si x C1–x alloys

Abstract: Results on the photoluminescence, optical properties and photoconductivity of amorphous Si : C : H alloys prepared from the plasma decomposition of silane and ethylene are presented. Films deposited at substrate temperatures of 30 and 300°C have been studied with compositions ranging from 100 to 10% silicon. The photoluminescence spectra measured at 77 and 300 K show considerable discrepancies with respect to previously reported results. The emission is in the form of a single band which broadens and shifts towards higher photon energies as the carbon concentration is increased. The highest emission-peak energy of 2·05 eV is achieved for samples with 90 at.% C. The temperature quenching of the luminescence is reduced with increasing carbon concentration. For 90% C specimens there is less than a factor of 2 reduction in emission intensity between 77 and 300 K. As a consequence visible (yellow–orange) room-temperature luminescence can be clearly observed with the naked eye. The luminescence efficie...

A model for the electronic transport in hydrogenated amorphous silicon

Abstract: We present a model for the steady-state transport in glow-discharge (g.d.)-deposited silicon that can account in detail for the ‘activated mobility’ observed from a comparison of thermopower and co...

Calculations of off-centre displacements of divalent substitutional ions in CaO, SrO and BaO from model potentials

Abstract: The problems of representing the polarizability of oxygen in potential models for oxides are discussed. Simple shell-model interionic potentials for MgO, CaO, SrO, BaO, MnO, FeO, CoO and NiO are proposed and the parameter fitted to the dielectric constants and lattice spacings. The oxygen polarizability and interactions are taken to be the same in all cases. Model predictions of cohesive energies, elastic constants and phonon dispersion are tested against experiment and found to be reasonably satisfactory for such an oversimplified model. Defect energies are calculated for Mg2+, Mn2+, Fe2+, Co2+ and Ni2+ substitutional impurities in CaO, SrO and BaO host crystals. Off-centre minima displaced in 〈111〉 directions are found in all cases in SrO and BaO, although SrO: Mn2+ is marginal: in CaO all the impurities studied remain on-centre. For systems which have been investigated experimentally these predictions show good agreement.

Demonstration of an ultra-high mobility organic polymer

Abstract: Experimental evidence is given for a high mobility (μ ∼ 20 m2 s−1 V−1) and a low field-saturated drift velocity (v d ∼ 2·2 × 103 m s−1) for carrier motion along a polymer chain. Photoconduction experiments in time domains from 10−9 to 102 seconds and fields from 102 V m−1 to 106 V m−1 are described. The polymer, the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol, is a single crystal in which the conjugated semiconducting carbon chains are all parallel to each other. Carriers travel a distance approaching 1 mm along a chain before trapping at a defect. It is possible to cut the chains and detect the resulting reduction of range of carrier motion.

Dispersive hopping transport via sites having a Gaussian distribution of energies

Abstract: The Monte Carlo technique has been employed to simulate hopping transport through a Gaussian energy distribution of hopping sites located on a cubic lattice having a nearest-neighbour distance of 10−7 cm. Dispersive transport is observed at temperatures where the width of the Gaussian [sgrave] > kT and for electric fields where eE x 10−7/[sgrave] < 1. The dispersive regime is observed over a considerable range in the average number of jumps and in this range both ⟨x⟩ and ⟨x 2⟩ can be described by a mobility and a diffusion constant which are proportional to t α–1 and where α is a function of electric field and [sgrave]/kT.

Chemical bonding states in the amorphous SixC1–x: H system studied by X-ray photoemission spectroscopy and infrared absorption spectra

Abstract: XPS studies and infrared absorption measurements of the reactively sputtered (RS) amorphous SixC1–x: H alloy system have been made. The binding energy of the Si 2p core electrons decreases monotonically as the alloy composition x increases while the corresponding line-width remains almost constant. On the other hand, a curve of the C 1s core electron binding energy versus x has a kink at around x = 0·5∼0·6. Infrared absorption spectra reveal the existence of C–H, Si–H, Si–C bonds in the films. These results are discussed in terms of chemical bonding states.

On the theory of a.c. conduction in amorphous semiconductors and chalcogenide glasses

Abstract: It is shown that electron–electron interaction changes the frequency and temperature dependences of the a.c. conductivity at low temperatures. The result is that the factor kT in the Austin–Mott formula has to be replaced by e 2/κr ω. The effect is similar to the enhancement of the density of states by electron–hole interaction in the interband optical absorption.

Molecular dynamics simulation of silicon dioxide glass

Abstract: A molecular dynamics study of the thermodynamic, structural and dynamical properties of a model silicon dioxide glass is described. The system consists of 3000 ions in a cubic volume. The results of the present simulation are compared with the results of our earlier simulation of a small system containing only 375 ions. The glass transition point is identified by a sudden change in the value of specific heat and pressure. Below this point diffusion also ceases. The structure of simulated glass is found to be a three-dimensional continuous network of SiO4 tetrahedra, connected at the oxygen nodes with a broad distribution of Si-O-Si angle centred around 153°. Other structural features, namely radial distribution functions and the interference function, are found to be in good agreement with the experimentally observed values. The holes in this open network system are interconnected, and form a continuous network. The existing inert gas solubility data in SiO2 glass are explained by using the hole ...

On the glass transition and the residual entropy of glasses

Abstract: A general statistical–mechanical description of the liquid–glass transition as a transition from ergodic to non-ergodic behaviour is presented. The description is based on the concept of the differ...

Transport properties of a-Si: H alloys prepared by r.f. sputtering

Abstract: Conductivity and thermopower measurements as a function of temperature are reported for a series of r.f. sputtered hydrogenated a-Si samples doped with P. The results are analysed, along with others in the literature, in terms of several models for transport. It is concluded that the best overall fit to the data involves conduction in an intrinsic band above about 400 K and conduction in a P-related impurity band between about 200–400 K. The proposed transport in the latter temperature regime disagrees with earlier conclusions of Spear and Le Comber, and it also implies a smaller shift of the Fermi level with doping than does their model. The implications of this interpretation for current measurement of Hall effect and drift mobility are briefly outlined, as are difficulties in the detailed interpretation and complications related to heterogeneity in structure.

The absolute scale of thermoelectricity II

Abstract: Direct measurements of the Thomson heat of lead have been made from 300 to 600 K, extending the author's previous work (Roberts 1977) from 0 to 350 K. Direct measurements for copper from 300 to 850 K make possible a convenient high-temperature scale which is shown to be more accurate than earlier scales. Thermoelectric measurements have also been made to provide compatible scales for gold and platinum. Deviations from the tables of Cusack and Kendall (1958) range from about – 0·1 μ V/K to + 0·8 μ V/K. Values from Rudnitskii (1956) are found to be in much better agreement with our scales.

The minimum metallic conductivity in three dimensions

Abstract: A minimum metallic conductivity ([sgrave]min) was proposed by the present author in 1972. In 1976 he showed that the existence or otherwise of this quantity depended on the index s, defined if the radius of an Anderson localized state at energy E tends to infinity as (E c–E)−s , where E c is the mobility edge. The quantity [sgrave]min exists only if s ⅔ (see also Stein and Krey 1979). Some theoretical treatments give s = ≥⅔, others give 0.6. Recent experiments suggest that there is a very narrow range of energies above E c in which, at T = 0, [sgrave] falls continuously to zero. This paper, therefore, starting from the Kubo–Greenwood formula, examines the consequences of the assumption that s = 0.6. It is found that, if this is so, there is indeed a narrow range in which [sgrave] at T = 0 climbs from zero to the calculated value of [sgrave]min; above [sgrave]min, [sgrave] is proportional to {N(E F)}2 as long as L ∼ a, where a is the distance between electron sites and L the mean free path. The te...

The electronic properties of silicon nitride

Abstract: The nature of bonding in silicon nitride is treated using simple bond-orbital models. A nitrogen pπ lone-pair valence band maximum is found. This is due to the planar nitrogen site, which is in turn due to the repulsions between non-bonded second-neighbour silicon atoms. The conduction minimum is found to have a relatively low effective mass and be formed of Si 3s states. The density of states (DOS) for β-Si3N4 is calculated for two Si–N–Si bond angles, 120° and 107°. The DOS at the former angle, which corresponds to a planar nitrogen, shows an upper pπ valence band which has merged into the lower bonding bands at 107°. The effects of non-bonded silicon–silicon repulsions on the planarity of the nitrogen site and likely structure of amorphous silicon nitride are discussed. Unlike vitreous SiO2, commercial silicon nitride contains an appreciable proportion of impurities which may determine electronic transport and low-energy optical properties. The local electronic structure of ≡Si–Si, ≡Si–H, =N–H...

Analysis of field-effect-conductance measurements on amorphous semiconductors

Abstract: A method for calculating the relationship between the density of localized states in an amorphous semiconductor and the experimentally measured field-effect conductance is presented. Commonly used simplifying assumptions of a constant space-charge density, zero-temperature statistics and a parabolic band-bending potential profile are removed and the errors that are introduced by using these approximations calculated. The method is used to show that the existence of a reported peak in the density of states at 0·4 eV below the conduction-band mobility edge of amorphous silicon (the Ex peak) cannot be proved through field-effect-conductance measurements.

Preparation and properties of amorphous phosphorus nitride prepared in a low-pressure plasma

Abstract: Thin films of amorphous P3N5 and P3N5Hx have been deposited in a low-pressure plasma either from the elements or from phosphine and nitrogen. Electric, optical and structural properties of these compounds are derived from measurements of the dark conductivity, of the I.R., optical and U.V. absorption, of Raman spectra and from studies using X-ray photoelectron spectrometry and differential scanning calorimetry. A structural model of the amorphous phosphorus nitride is presented.

The Hall effect in the variable-range-hopping regime

Abstract: The calculated Hall mobility μH due to hopping transport in spatially random systems is extended to include site-energy disorder. In the low-temperature, variable-range-hopping limit, an upper limit to μH is found and its temperature dependence is established.

An improved field-effect analysis for the determination of the pseudogap-state density in amorphous semiconductors

Abstract: A method is described for determining the gap-state density N(e) of an amorphous semiconductor from field-effect data, in which no assumptions are made about the form of the band-bending in the semiconductor. The problem is reduced to three successive integrations over an assumed N(e) by change of variable from depth x to voltage V, and the best fit to the experimental data is obtained by iteration of the assumed state density. The method is shown to be no less rigorous and considerably more economical than the recent analysis of Goodman, Fritzsche and Ozaki (1980). In addition, we demonstrate an experimental means of determining flat-band conditions to an accuracy of 1/2 kT of band bending, by finding the value of V g for which (kT/e)d log I SD/dV g is independent of temperature.

Surface characterization of uranium metal and uranium dioxide using X-ray photoelectron spectroscopy

Abstract: X-ray photoelectron spectra of pure uranium metal and stoichiometric uranium dioxide have been obtained using an AEI ES300 spectrometer. Binding energy values for core and valence electrons have been determined using an internally calibrated energy scale and monochromatic Al Kα radiation. Satellite peaks observed accompanying certain principal core ionizations are discussed in relation to the mechanisms by which they arise. Confirmation is obtained that for stoichiometric UO2·00 a single shake-up satellite is observed accompanying the U 4f7/2,5/2 principal core lines, separated by 6·8 eV to higher binding energy.

Structure of molten alkali chlorides

Abstract: The structure of molten alkali chlorides has been studied by molecular dynamics simulation. A comparison is made of rigid-ion and shell models of the interionic potentials. It is shown that the effect of the polarizability of the ions is small when the two ions have similar sizes but is substantial when the cation is relatively small and unpolarizable. For NaCl the use of polarizable-ion potentials yields significantly better agreement with experiment whereas for RbCl it does not appear to be important for the structure. The excess number and excess charge fluctuations for the polarizable-ion model of NaCl are presented.

Photocarrier creation in one dimension

Abstract: Experimental results, and theoretical discussion, are presented on photocarrier creation in large single crystals of a conjugated polymer: the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-d...

The electronic structure of weak, random, two- and three-dimensional potentials

Abstract: The electronic structure of weak, random, two- and three-dimensional potentials is calculated by transforming the Hamiltonian to a chain model. The eigenvalues and eigenvectors are calculated for the chain model. In two dimensions all states are localized either exponentially at the band edges or more weakly at the band centre. In three dimensions the states at the band edges are also exponentially localized, but are extended at the band centre. The proportion of states localized, the size of localized states, the position of mobility edges and the density of extended states near the mobility edge are all calculated.

Simulation of carrier transport and energy relaxation in a macroscopic hopping system of sites with a Gaussian energy distribution

Abstract: The transport properties within a cubic lattice consisting of 70 × 30 × 30 hopping sites subject to a Gaussian energy distribution [sgrave]=0.1 eV wide, have been simulated by a Monte Carlo technique. Transit pulse shapes, the time evolution of the energy of the hopping carrier and number of new sites visited as a function of time are analysed. It is found that within the time regime of dispersive transport the energy loss per carrier per jump is inversely proportional to the previous number of new sites visited. This is consistent with a t α-1 behaviour of the jump rate. After a time τe, dynamic equilibrium is established leading to Gaussian transport. For disordered molecular solids in which hopping transport dominates and which are characterized by [sgrave]/kT 0.5, τe is found to be less than 10−8 s. This value would prevent detection of the dispersive regime employing conventional electrical circuitry. The mean carrier displacement at t=τe is typically ten lattice planes. In the...

Disorder effects and the optical properties of amorphous GaAs and GaP

Abstract: The optical properties of flash-evaporated amorphous GaAs and GaP thin films have been determined between 0.5 and 6.20 eV as a function of deposition conditions and heat treatments. The data obtained for stoichiometric samples of each compound are compared with each other and with similar data obtained earlier on evaporated amorphous Ge films. The effects of deviations from stoichiometry are also considered. These results are discussed and interpreted in relation to the results of complementary experiments, for example electrical conductivity. The nature of the defects responsible for the observed properties is discussed for both compounds, special attention being paid to the possibility of like-atom bonds.

Complex-refractive-index profiles of 4 MeV Ge ion-irradiation damage in silicon

Abstract: A new optical method is introduced for the determination of optical depth profiles in silicon. Alterations of the refractive index n and of the extinction coefficient k can be resolved down to 5 × 10−4 with a depth resolution of 20 nm. The case of 4 MeV Ge ion-irradiation damage has been chosen to demonstrate the way how n and k are evaluated from reflection and transmission profiles measured at the bevelled surface of silicon samples. Wedge interference fringes in the reflection profile are used to determine the photon energy dependent optical constants of ion-irradiated amorphous silicon between 1.1 and 2.5 eV. The different dose dependence (2 × 1012–1 × 1016 Ge ions cm−2) of the n and k profiles at a photon energy of 1.28 eV is interpreted by the irradiation induced generation of different types of defects such as amorphous clusters and defectively coordinated silicon bonds.

Gyromagnetic remanence and anisotropy in single-domain particles, rocks, and magnetic recording tape

Abstract: Experimental results are described for the gyromagnetic remanence acquired by two rock samples and a sample of magnetic tape arbitrarily orientated in a non-magnetic holder. The remanence (gyroremanent magnetization—GRM) is produced in standard ‘demagnetization’ equipment when the sample is exposed without rotation to a strong alternating field in the absence of any other fields. This effect has already been qualitatively explained (Stephenson 1980 c) in terms of an effective transient biasing field resulting from the intrinsic angular momentum associated with the moments of single-domain particles as they are forced to undergo irreversible flips in the strong applied field. By constructing a theoretical model of a three-dimensional anisotropic distribution of uniaxial particles, a consideration of the average flip of the assembly using this transient field approach enables expressions for the dependence of GRM on field-axis orientation to be derived. Use of these expressions in conjunction with ...

Multiple charge-states of transition-metal impurities

Abstract: Transition metal ions in MgO and CaO are observed in several charge states. We investigate the conditions for stability of the observed ions of manganese, iron, cobalt and nickel. These lead to two main conclusions. First, the stability of so many states results from a balance between the variation of ionization potential with charge state and the dependence on excess charge of the polarization and distortion of the host. Covalency is unimportant, contrary to mechanisms discussed for gold in germanium. Secondly, the detailed quantitative calculations indicate that the full set of experimental data require that these oxides have a negative electron affinity, probably more negative than – 1·3 eV in MgO and more than −0·9 eV in CaO.

Small-polaron absorption in W x Mo1–x O3

Abstract: Optical absorption and reflection spectra of WO3, WO3 : H0, W x Mo1–x O3 : H0, and sub-stoichiometric ternary oxides have been measured. In WO3, the free-carrier absorption at room temperature switches over to small (or intermediate) polaron absorption near 0·7 eV at low temperatures. In fully oxidized ternary oxides doped with hydrogen, quasi-localized charge carriers with optical transition energies between 1·24 and 1·42 eV have been found. Sub-stoichiometric compounds with extended defect structures like crystallographic shear planes or tunnel structure reveal intervalence transitions including Mo5+, W5+, Mo6+ and W6+ states. An energy level scheme is discussed including the effects of metal–hydrogen interaction.

Resistivity mechanisms during clustering in alloys

Abstract: It is argued that as a binary alloy ages, two distinct mechanisms are responsible for the decrease in electrical resistance which follows the initial increase. Each mechanism has the malicious effect of making the other appear unimportant, and this has caused some misunderstanding in the literature. The arguments are demonstrated in a calculation for an AlZn alloy.

Transport properties, magnetic susceptibility and Mössbauer spectroscopy of Fe0.25NbS2 and Fe0.33NbS2

Abstract: Resistivity, Hall voltage, magnetic susceptibility and Mossbauer parameters of Fe0·25NbS2 and Fe0·33NbS2 have been measured on powders and on single crystals as a function of the temperature, from ...